Benzene being a toxic component, its concentration in gasoline blends is a major health concern and hence placed under environmental regulations worldwide. Current regulations restrict the annual average benzene level in Gasoline sold in U.S to 0.62% vol. The major contributors of benzene in the gasoline pool in the hydrocarbon industry, typically, are reformate, hydrogenated pyrolysis gasoline (PG) and catalytically cracked gasoline.
Removal or recovery of benzene from reformate and hydrogenated PG is straightforward and is carried out by solvent extraction and/or extractive distillation using polar solvents such as NMP, Sulfolane, NFM, etc., and several commercial units are currently in operation worldwide. A number of patents are available describing these processes. For example, U.S. Pat. No. 3,591,490 shows a process for separating aromatic hydrocarbons from reformate and hydrogenated pyrolysis gasoline using N-Methyl-2-Pyrrolidone (NMP) or Di-methylformamide (DMF) as a solvent. Similarly, U.S. Pat. Nos. 3,723,256 and 5,022,981 disclose methods of recovering aromatics from hydrogenated pyrolysis gasoline with sulfolane or other related solvents using extractive distillation. However, these patents deal with hydrocarbon mixtures like reformate and hydrogenated pyrolysis gasoline and do not cover treatment of cracked feedstocks.
Unlike reformate and hydrogenated PG, unprocessed cracked gasoline fraction contains olefins along with impurities like oxygenates, metals, chlorides, sulphur compounds, nitrogen compounds, and organic peroxides. Due to the complex nature of this feedstock, an economic and reliable benzene recovery process is difficult to develop and has not been practiced in the industry so far.
Both olefins and aromatics in cracked gasoline contribute substantially to the octane number in the gasoline pool. An attempt to reduce benzene by well-known hydro-processing routes would result in saturating the olefins as well, thus lowering the octane of the cracked gasoline fraction. Several other alternative methods have also been developed to reduce benzene in cracked gasoline. Some of these are described below.
One such process removes benzene from FCC naphtha stream containing paraffins, C6 olefins and C6 iso-olefins. This process includes a number of steps such as:                Separation of benzene concentrate stream;        Subjecting this stream to etherification with an alcohol over an etherification catalyst to convert the C6 iso-olefins to ethers;        Separating the ethers of C6 iso-olefins from benzene concentrate;        Dissociation of ethers of C6 iso-olefins to recover alcohol and C6 iso-olefins; and        Hydro treatment of ether removed benzene concentrate to remove olefins and organic impurities.        
Removal of benzene from hydro treated benzene concentrate fraction using solvent extraction. The main emphasis is given on the etherification of iso-olefins and their separation from benzene. The process incorporates a number of steps for benzene removal and no commercial units came out based on the process mentioned. Further, the removal of benzene by solvent extraction from hydro-treated benzene concentrate underlines the difficulty in the recovery of benzene from olefinic feedstock containing substantial impurities.
Another patent, U.S. Pat. No. 8,143,466, discloses a process for removal of benzene from gasoline and involves partial alkylation of benzene in presence of catalyst with alcohol and ether. Alkylated benzene is recovered as bottom stream and the top hydrocarbon stream is water washed to recover the un-reacted alcohol and ether.
Other processes for removal of benzene from benzene rich hydrocarbon fraction have been developed and commercialized that involve alkylation of benzene with light olefins rich feed stock over a solid acid catalyst. But this process is applied to reformate and cannot be used for treating unprocessed cracked gasoline because of the susceptibility of the catalyst to the impurities present in cracked gasoline
The above processes either convert benzene or involve several steps to remove benzene from cracked gasoline fraction.
As mentioned above, there is no commercial unit operating for benzene recovery from unprocessed cracked gasoline fraction (boiling in the range 40 to 90° C.) containing olefins, di-olefins, paraffins, iso-paraffins, naphthenes, benzene, along with impurities like oxygenates, metals, chlorides, sulphur compounds, nitrogen compounds, and organic peroxides. One of the reasons may be the potential of polymerization of olefins, particularly di-olefins (specially conjugated types) in the presence of reactive organic peroxides.
Instances of such polymerization, especially when conjugated olefins are present, have been reported many a time as evident below.
A number of investigators have found that di-olefins containing conjugated double bonds oxidize and produce organic peroxides much more readily than olefins of other types. It has been shown that fulvenes absorb oxygen and resinify with extraordinary rapidity. Conjugated di-olefins oxidize much more readily than simple olefins, and it has been reported that accelerated oxidation tests do not affect simple olefins, but only di-olefins, and that di-olefins as a class are markedly less stable than mono olefins, but the position of the double bonds is important in determining stability. Conjugated double bonds introduce extreme instability, while compounds containing double bonds widely separated is almost as stable as an average olefin. Similar conclusions have been drawn regarding hexadienes. Those with separated double bonds did not absorb a measurable amount of oxygen during several months' exposure at room temperature. The isomeric conjugated compounds have been shown to absorb oxygen immediately on exposure, and that oxidation continues at a rapid rate.
The tendency of cracked gasoline to react with oxygen and form peroxides has been found to be attributable in part to conjugated di-olefins and in part to other olefinic material. Di-olefins and olefins present together form more peroxidic compounds and more gum than when present individually in the same concentration.
It has also been reported that peroxides develop in stored gasoline. The actual structures of the peroxidic substances formed by auto oxidation have not been ascertained nor their formation chemistry. Oxygenates like aldehydes have been reported in oxidized cracked gasoline and it is postulated that some of the resulting peroxides may originate from them as well. It is also theorised that per-acids formed from aldehydes are also essential catalysts in polymerization.
Alkenes are known to undergo polymerization at high reaction temperature in polar medium under acidic conditions. Alkenes with more than 2 carbons have reactive allylic carbon atoms which in turn have allylic hydrogen atoms. Allylic carbon-hydrogen bond dissociation energy is relatively less than other C—H bond energies due to which allylic hydrogen can be substituted relatively easily. The resonance stabilisation of the formed allylic radical/cation/anion is the main factor responsible for substitution of allylic hydrogen. In a free radical substitution reaction the allylic radical formed can be stabilized by resonance. Thus conjugated di-olefins are more susceptible to oxidation. In presence of free radicals at high temperature or even in presence of di-radical oxygen, these allylic carbon-hydrogen bonds generate allylic radicals which are subsequently stabilized by resonance. The allylic radicals attack other olefin molecules and initiate chain growth polymerization. These free radicals may also react with di-radical oxygen to give peroxy radicals through auto-oxidation reactions. These peroxy radicals can extract hydrogen from olefin molecules to yield hydro-peroxides and generate new allylic free radical giving a chain reaction.
It has also been theorised that the initial products of oxidation of unsaturated hydrocarbons are peroxides which eventually end up forming acids, mainly found in the end polymerized products.
Gasoline gum can also originate from oxidation of both reactive hydrocarbons and gasoline impurities (non-hydrocarbons). Paraffins, aromatics, mono-olefins and di-olefins are increasingly unstable towards oxidation. Polymerization and gum formation in gasoline can result from the combined oxidation of reactive hydrocarbons and impurities. Certain metals in small concentrations have also been reported to catalyze the deterioration of gasoline.